Embodiments of the present invention provide a rotational gantry designed to provide proton radiation therapy using a mono-energetic proton beam. The mono-energetic proton beam is transported by a beam line transport system having two or more bending magnets and a plurality of quadrupole and steerer magnets for directing and focusing the proton beam. Energy variation of the beam is performed directly before the beam reaches an isocenter of the gantry.
